Cement and concrete are relatively brittle materials with low tensile strength. In attempts to overcome this deficiency of cement and concrete materials, randomly oriented, short discontinuous fibers have been introduced into the cementitious material as a reinforcing material for the cementitious matrix. However, addition of fibers tends to increase the viscosity of the cementitious matrix and to render the material difficult to handle and place. For this reason, in bulk construction using conventional mixing techniques/equipment, only short fibers (e.g. 25 millimeters length) and low fiber volume fractions (e.g. less than 1%) have been used heretofore. For such reinforced cementitious materials, the fibers do not significantly influence the tensile strength of the matrix. Only after the matrix has cracked do the fibers contribute to strength by bridging existing cracks.
Several techniques are used to make commercial fiber-reinforced cement products. The known Hatschek process was initially developed for-production of asbestos composites and is now utilized for manufacture of non-asbestos, short discontinuous fiber (e.g. wood fibers and/or polyethylene pulp) reinforced cement composites. In this process, a fiber-cement mixture with excess water is deposited (e.g. roll coated) on a felt band substrate, vacuum dewatered, calendared, and cured to form a fiber reinforced cement matrix in sheet form. However, this method is suitable only for fiber types which retain cement particles during vacuum dewatering. Composites made by the Hatschek process are brittle and only good for sheet.
U.S. Pat. No. 5,108,679 describes manufacture of discontinuous fiber-reinforced cement roofing products using the known roller and slipper process. In this process, the premixed materials including not more than 4 weight % fibers are compressed by passage through rollers and then slipper to obtain flat reinforced sheets to which the process is limited.
Other manufacturing techniques for fiber-reinforced cement products employ continuous fibers rather than short, discontinuous fibers. For example, the known Reticem process produces cement laminate composites with 20 to 30 continuous fiber mesh layers. In particular, each fiber mesh layer is fed from a mesh supply reel, spray coated with cement, covered with the next mesh layer that is then spray coated with cement and so on to form the multi-layered laminate that is compacted, trimmed to length, and cured.
The known pultrusion process produces continuous fiber-reinforced structural shapes with very high fiber volume ratios. In particular, in practice of the pultrusion process, continuous fiber mats are fed from stationary and roving mat creels to a cement slurry bath for coating. Then, the coated mats are formed to shape and cured under pressure.
The Reticem and pultrusion processes described above are disadvantageous in that they require continuous fibers and the processing/equipment technology for incorporating the continuous aligned fibers in the cement matrix are costly. As a result, these processes have been used for the most part in the manufacture of specialty products, such as thin sheets.
U.S. Pat. No. 4,066,723 describes production of laminated cement sheets wherein an unreinforced sheet of concrete is extruded, reinforcing fibers are then distributed onto the surface of the sheet, and these steps are repeated to produce a flat lamination. The process is limited to production of flat laminated products.
It is an object of the present invention to provide a method of making discontinuous fiber reinforced cement matrix composites, as well the reinforced cement composites themselves, using relatively high volume fractions of discontinuous reinforcing fibers by die extrusion to provide improved mechanical properties in that direction.